When did the Roman Empire end?

The Roman Empire did not disintegrate all at once, so there are several parts to the answer to your question. The Roman Empire as a single, unified entity ended in 285 AD. Theodosius the Great briefly reunited it during his reign, but it remained permanently divided following his death in 395. From then on, the Western Roman Empire was ruled from Rome, and the Eastern Roman Empire was ruled from Constantinople.

The Western Roman Empire suffered an onslaught from barbarian invaders and eventually collapsed in 476 AD, when Emperor Romulus Augustulus was forced to abdicate the throne.

The Eastern Roman Empire lasted for almost 1,000 years after the Western half fell. Eventually, however, the Eastern Roman Empire succumbed to a perpetual assault from Ottoman Turks. It fell in 1453.